What's the difference between Bias and Distance?
Distance is the distance between the High poly object and the low poly object (in blender unit).   



Note that the distance is only taken into account on the Z axis , and the low poly mesh must be directly under the high poly mesh.
if you do any of the above wrong, the result will be an empty baked image. 

Setting the distance too low will make your details not appear on your final baked image since the rays will die halfway before they reach the low poly mesh.   




The bias is only related to the high poly object, it controls how much of the geometry is preserved , anything after the bias is cut off.   

The black lines are where the rays start being fired down to the low poly object, the rays start from lowest vertex of the high poly object.The rays' length is determined by the distance value.  

The red line is the bias distance,it also starts from the lowest vertext of the high poly object, but is fired to the opposite direction(upward).  

The details between the black line and the red line are preserved and will appear on the resulting baked image.  

The details after the red line (greater than the bias, shown in blue) are lost , and will appear on the baked image as if that area was a flat surface.  



Setting the bias value correctly is important when the high poly mesh has curved,round surfaces.   


Please feel free to edit this answer and correct it.